The present invention relates to hydraulic control valves, for example, for use in hydraulic power steering devices for motor vehicles.
Motor vehicles, especially trucks and like large-sized vehicles, are equipped with a hydraulic power steering device for assisting the driver in the steering force. Such power steering devices generally have a hydraulic cylinder for assisting in the steering force, and a rotary-type or spool-type hydraulic control valve for changing the direction of pressure oil to be supplied thereto according to the steering direction (see Unexamined Japanese Patent Publication SHO 64-22676 and Examined Japanese Patent Publication SHO 57-61620). FIGS. 10 to 12 show an example of conventional rotary-type hydraulic control valve.
With reference to FIG. 10, the hydraulic control valve comprises a cylindrical casing 1, and a cylindrical valve member 2 fitted in the casing 1 and rotatable about the axis thereof.
When the hydraulic control valve is used for a power steering device of the rack-pinion type, the casing 1 is rotatably supported by the casing of the steering device although not shown. The valve member 2 is formed at the lower end of an input shaft connected to the steering wheel. The casing 1 is secured to the upper end of an output shaft connected to the pinion shaft. The input shaft and the output shaft are interconnected by a torsion bar 60 so as to be twistable relative to each other.
The inner periphery of the casing 1 is formed with eight rectangular groove portions 4, 5 and eight rectangular land portions 6, 7 extending axially of the casing and arranged alternately circumferentially thereof as will be described below. Four first land portions 6 are formed at positions dividing the circumference of the inner periphery of the casing 1 into four equal portions, and four second land portions 7 are formed between the first land portions. Four first groove portions 4 are formed clockwise adjacent to the respective first land portions 6. Four second groove portions 5 are formed counterclockwise adjacent to the first land portions 6, respectively. The outer periphery of the valve member 2 is formed with eight rectangular groove portions 8, 9 and eight rectangular land portions 10, 11 extending axially of the member and arranged alternately circumferentially thereof in the following manner. Four first groove portions 8 are formed at positions dividing the circumference of the outer periphery of the valve member 2 into four equal portions and are opposed to the respective first land portions 6 of the casing 1, and four second groove portions 9 are formed between the first groove portions 8 and opposed to the respective second land portions 7 of the casing 1. Clockwise adjacent to the first groove portions 8, four first land portions 10 are provided as opposed to the respective first groove portions 4 of the casing 1. Counterclockwise adjacent to the first groove portions 8, four second land portions 11 are provided as opposed to the second groove portions 5 of the casing 1.
Although not shown, the inner periphery of housing of the power steering device in contact with the outer periphery of the casing 1 is formed with three independent annular grooves. Formed in the casing 1 are four radial oil inlet ports (oil inlet channels) 12 extending through the respective first land portions 6 from the inner periphery to the outer periphery, four first oil outlet ports (oil outlet channels) 13 extending through the respective first groove portions 4 from the bottom thereof to the outer periphery, and four second oil outlet ports (oil outlet channels) 14 extending through the respective second groove portions 5 from the bottom thereof to the outer periphery. The oil inlet port 12, the first oil outlet port 13 and the second oil outlet port 14 are positioned as displaced from one another axially of the casing. Through the first of the annular grooves of the steering device housing, the four oil inlet ports 12 are in communication with a hydraulic pump (not shown) which is driven by an engine. Through the second annular groove of the housing, the four first oil outlet ports 13 communicate with a first oil chamber of a hydraulic cylinder (not shown) for assisting in the steering force. Via the third annular groove of the housing, the four second oil outlet ports 14 communicate with a second oil chamber of the hydraulic cylinder. The cylinder has a piston secured to a rack bar. When supplied with pressure oil to the first oil chamber, the hydraulic cylinder produces a rightward steering force. The supply of pressure oil to the second oil chamber produces a leftward steering force.
A first return oil bore (return oil channel)15 extends through the center of the valve member 2 axially thereof and is in communication with the four second groove portions 9 through four radial second return oil bores (return oil channels) 16. Although not shown, the first return oil bore 15 communicates with an oil tank. The torsion bar 60 interconnecting the input shaft and the output shaft extends through the first return oil bore 15.
While the steering wheel is not manipulated, the torsion bar 60 is free of torsion, with the input shaft and the output shaft, i.e., the hydraulic control valve, in a neutral state. This state is illustrated in FIG. 10, in which the first land portions 6 of the casing 1 are opposed to the first groove portions 8 of the valve member 2, respectively, the second land portions 7 of the casing 1 to the second groove portions 9 of the valve member 2, the first groove portions 4 of the casing 1 to the first land portions 10 of the valve member 2, and the second groove portions 5 of the casing 1 to the second land portions 11 of the valve member 2. There is a clearance between each of the first land portions 6 and the second land portions 7 of the casing 1 and each of the valve member first land portion 10 and second land portion 11 on opposite sides thereof. The oil supplied to each oil inlet port 12 from the hydraulic pump enters the first groove portion 8 of the valve member 2, flows through the casing first groove portion 4 and second groove portion 5 on opposite sides thereof, through the valve member second groove portions 9 on opposite sides thereof and further through the second retrun oil bores 16 of these portions, enters the first return oil bore 15 and returns to the tank. Accordingly, no pressure oil is supplied to the hydraulic cylinder, permitting the rack bar to remain in a neutral state and the motor vehicle in a straight running state.
When the steering wheel is turned rightward, the torsion bar 60 is twisted, slightly rotating the valve member 2 clockwise relative to the casing 1 as seen in FIG. 11. As a result, the first land portion 10 and the second land portion 11 of the valve member 2, and the second land portion 7 and the first land portion 6 of the casing 1 block communication between the first groove portion 8 of the valve member 2 and the second groove portions 9 on opposite sides thereof, with the first groove portion 8 of the valve member 2 in communication with the first groove portion 4 of the casing 1 and with the second groove portions 9 of the valve member 2 in communication with the second groove portions 5 of the casing 1. The oil supplied from the oil inlet port 12 to the first groove portion 8 of the valve member 2 is fed to the first oil chamber of the hydraulic cylinder via the first groove portion 4 of the casing 1 and the first oil outlet port 13. On the other hand, the oil in the second oil chamber of the hydraulic cylinder flows out through each second oil outlet port 14 into the second groove portion 5 of the casing 1, passes through the second groove portion 9 and the second return oil bore 16 of the valve member 2 into the first return oil bore 15 and returns to the tank. Consequently, the rack bar moves in a direction to produce a rightward steering force and steers the motor vehicle rightward.
Conversely when the steering wheel is turned leftward, the torsion bar 60 is twisted in a direction opposite to the above, slightly rotating the valve member 2 counterclockwise relative to the casing 1 as shown in FIG. 12. Consequently, the first land portion 10 and the second land portion 11 of the valve member 2, and the first land portion 6 and the second land portion 7 of the casing 1 block communication between the first groove portion 8 of the valve member 2 and the second groove portions 9 on opposite sides thereof, with the first groove portion 8 of the valve member 2 in communication with the second groove portion 5 of the casing 1 and with the second groove portions 9 of the valve member 2 in communication with the first groove portions 4 of the casing. The oil from the oil inlet portion 12 to the first groove portion 8 of the valve member 2 is fed to the second oil chamber of the hydraulic cylinder via the second groove portion 5 of the casing 1 and the second oil outlet port 14. On the other hand, the oil in the first oil chamber of the hydraulic cylinder flows out through each first oil outlet port 13 into the first groove portion 4 of the casing 1, passes through the second groove portion 9 and the second return oil bore 16 of the valve member 2 into the first return oil bore 15 and returns to the tank. Accordingly, the rack bar moves in a direction to produce a leftward steering force and steers the motor vehicle leftward.
In the case of the power steering device provided with the hydraulic control valve described above, the rack bar of the hydraulic cylinder remains subjected to the hydraulic assisting steering force if the steering wheel is held turned rightward or leftward to a limit position. The assisting steering force is great especially in the case of trucks or large-sized vehicles, so that the linkage of the steering device is likely to break when the steering wheel is held as turned to the limit position.
As a countermeasure against this problem, a hydraulic power steering device has been proposed which is adapted to interrupt a high hydraulic pressure at the stroke end of piston of the hydraulic cylinder (see Unexamined Japanese Utility Model Publication SHO 60-122277). In actuality, however, it is likely that a maximum of assisting steering force will be maintained to break the linkage if the wheel falls into a trench or the vehicle is steered on a bad road even when the steering wheel is not turned to the limit. Thus, the problem remains unsolved since the proposed device is not adapted to interrupt the high hydraulic pressure unless the piston of the hydraulic cylinder is moved to the stroke end.
The same problem as above is also encountered with the power steering device having a hydraulic control valve of the conventional spool type.